PRESSURE AND TEMPERATURE DEPENDENCE OF THE NEAR UV ABSORPTION CROSS SECTION OF $HNO_{3}$

Abstract

In the atmosphere, $HNO_{3}$ is an important reservoir species for the odd nitrogen $(NO_{x})$ and odd hydrogen $(HO_{z})$ families, both of which contribute to the destruction of stratospheric ozone. In the sunlit atmosphere, $NO_{2}$ and OH are liberated by photolysis of $HNO_{3}$. Therefore, in order to model the distribution of these species, accurate values for the photodissociation cross section of $HNO_{3}$ are necessary, particularly for wavelengths beyond 290 nm, which dominate the available light in the lower stratosphere. Previous $investigations^{h}$ show significant discrepancies at wavelengths greater than 310 nm, and have also noted a marked temperature dependence in this region. In an effort to resolve these discrepancies, and determine if clusters of $HNO_{3}$ are affecting the previous measurements, we have reexamined the near UV absorbance cross section from 240 to 350 nm at a wide range of pressures and temperatures. Implications of these results for the observed diurnal variation of [OH] in the lower stratosphere will be discussed.